The technical field of this invention is material analysis and, in particular, the invention relates to the determination of specific properties of hydrocarbon-containing fluids, such as the octane rating of gasolines.
Gasoline fuels are characterized by a series of physical properties, such as the octane number, the Reid Vapor Pressure (RVP), and the aromatic content. These fuels consist of a mixture of hundreds of hydrocarbon compounds, and the fuel properties are related to chemical grouping of these components. For example, the octane number of an unknown fuel may be related to the volume percent of monocyclic aromatics blended into the mixture. This particular property of gasolines, the octane number, plays a rather important role at the consumer level: car makers recommend gasolines of certain octane rating for their vehicles, and such ratings at the gas station pumps guide the choices of consumers.
Unfortunately, at present, there is no system to determine quickly and inexpensively the octane rating of gasolines or other gasoline properties, such as Reid vapor pressure and aromatic content.
In particular, the octane rating for finished gasolines is traditionally determined by an involved and expensive laboratory test in which the gasoline sample is run through a test engine under specified conditions referred to as "Research" (RON) and "Motor" (MON); the final octane assignment is reported to the consumer at the pump as (RON+MON)/2, the average of these two measurements. Despite the cumbersome and labor-intensive nature of this octane test, it remains the mainstay of octane verification for gasoline fuels both at the research and the consumer level.
Other fuel properties can be estimated by physical measurements, but ultimately these properties are, likewise, attributable to the chemical composition of the fuel.
Conventionally, one approach to determining the chemical composition of compounds, such as hydrocarbon fuels, has been to apply techniques, such as Infrared spectroscopy and Raman spectroscopy. These spectroscopic techniques can often characterize the majority of the organic compounds which make up a hydrocarbon fuel with some precision. Full chemical analysis of a fuel sample can lead to a complete characterization of all fuel properties, provided that all fuel components are accurately measured in their proper proportions. However, such a procedure is often very time-consuming and, in some instances, impossible by using a single analytical approach.
Raman spectroscopy provides direct information on the vibrational states of the molecules in the substance. These vibrational states, as revealed from the main features of the spectrum, provide the "signature" of the different molecules in a mixture; and the intensity of the "peaks" in the spectrum relates to the number of molecules in a particular vibrational state. From that information, the relative abundance of different molecular compounds in a mixture can be ascertained with high precision. See, for example, U.S. Pat. No. 2,527,121 which discloses the use of Raman scattering techniques to determine the aromatic content of hydrocarbon mixtures.
Conventional Raman spectroscopy usually required the use of large and complex instruments and required long exposure times in order to obtain a reliable spectrum. The introduction of lasers into Raman spectroscopy has lessened the problem of exposure times and has also provided enhanced spectral resolution. As such, laser Raman spectroscopy has been used for several years in analytical chemistry as a highly discriminating analytical tool, taking advantage of the high intensities delivered by laser sources. For this reason, it is also especially well-suited for microliter quantities of liquid samples. Moreover, high resolution Raman spectra can often be obtained in minutes over a wide frequency range to give quick and reliable identification of chemical components in a mixture, based on their vibrational spectral features. Nonetheless, such high resolution spectra still require the use of large and complex laser sources and spectrometers, making it almost impossible to adapt these techniques to field measurements of fuel properties.
There exists a need for low cost, portable fuel property measuring systems that do not depend on large and highly complex mechanical devices. Moreover, there exists the need of a system that exploits the chemical analysis of the fuel properties without the complexity and bulkiness of the usual spectroscopic systems.